1. Field of the Invention
The present invention relates to a transmission power control method and a mobile communication system which can be applicable to a CDMA system (especially, a DS-CDMA transmission system) wherein the multiple-access can be performed by using spread spectrum in mobile communication.
2. Description of the Related Art
A conventional CDMA transmission system, such as a DS-CDMA transmission system, is a method in which secondary modulation for spreading a modulated information data signal by spread code at a high-speed rate is performed so that a plurality of communicators can communicate each other by using a common frequency band and each communicator is identified based on the spread code.
Therefore, in a receiver, it is necessary a received wide-band input signal must be converted to a narrow-band signal through inverse spread process before performing conventional demodulation process. In the inverse spread process of the receiver, the correlation of a received signal with a spread code replica synchronized with the spread code phase of the received signal is detected. In the DS-CDMA transmission system, since all the communicators use a common frequency band, the capacity for the subscribers is dependent on “SIR” (the ratio between the received signal power for desired wave and the interference power necessary for obtaining the predetermined receive error rate).
The problem when the DS-CDMA transmission system is applied to mobile communication is so-called “perspective problem” characterized by that the level of a signal received by a base station vary largely according to each mobile station (i.e., a signal of the mobile station near to the base station is received at higher power), and that a received signal at a high level in the mobile station interferes with a received signals from other mobile stations, causing the deterioration of reception quality.
A solution as to the above perspective problem is that the transmission power control has been proposed. In general, in the transmission power control method, transmission power of each mobile station is controlled so that the received signal power from the each mobile station received by the base station or SIR (the ratio between the received signal power and the interference power) is given regardless of the position of each mobile station, thereby obtaining the equitable communication quality within a service area.
[Problems of Prior Art]
The mobile communication cellular services, which utilize the DS-CDMA that commercial service has been put and has been published in a few years, are as follows: IS-95 [TIA/EIA/IS-95 “Mobile Station-Base Station Compatibility Standard for Dual-Mode Wide band Spread Spectrum Cellular System”, “Telecommunication Industry Association, July 1993.], W-CDMA [F. Adachi, M. Sawahashi, and H. Suda, “Wideband DS-CDMA for Next Generation Mobile Communication System” IEEE Commun. Mag., Vol. 36, pp. 56–69, Sept. 1998] and the like.
In these systems, in order to improve the performance of each system, channel coding (error correcting code), rake diversity reception method or the coherent demodulation method by using the pilot signal as well as the previous mentioned transmission power control can be used.
The performances of the channel coding, the rake diversity reception and the coherent demodulation by using the pilot signal vary depending on the number of multipath in the actual propagation path and the speed of the mobile station (phasing variation rate) or the like. For instance, in general, the effect of the channel coding increases relative to the increase of the speed of the mobile station. Further, on the contrary, in the case of the coherent demodulation using the pilot signal, the increase of the speed of the mobile station causes degradation in the accuracy of the amplitude and the phase of the received signal using the pilot signal, resulting in worsening of the reception error rate after demodulation.
Thus, in actual propagation path, even though the received signal power or the SIR is controlled to a fixed value by means of the transmission power control, the effect of channel coding, the effect of rake diversity effect, the accuracies of the estimation of amplitude and phase of the received signal using the pilot signal varies according to time variations such as the number of multipath and the velocity of the mobile station (phasing variation velocity), and so it becomes impossible to maintain a constant reception quality (reception bit error rate or reception frame error rate).
Further, in the case of the mobile communication cellular service using the DS-CDMA, one of the above literatures (IS-95, W-CDMA, etc.), the soft handover (site diversity) in which the hit-free high-quality reception can be realized is used.
In this case, for the up-link (a link comprising a mobile station as being a transmission side and a base station as being a reception side), the received signal in each base station is synthesized by radio network controller (RNC) as a station of higher hierarchy, thus improving reception quality. Therefore, when the target of the received signal power based on the transmission power control by each base station or the SIR is constant, the reception quality without side diversity becomes worse than that with the side diversity.